Actor
An Actor is responsible for performing an action, while a Subject is the thing that the action, observation, evaluation, or other expression concerns.
Specification-sourced: quoted from the governing specification, pending the applicable specification-owner approvals. The specification controls.
In RSMOne's words
A laboratory Organization may act as Observer while a soil LivingSystem is the Subject of its Observation. Whether an Actor may perform a given action is a separate question of agency and authority.
What it is not
Actor ≠ Subject
An Actor is the entity responsible for performing the action; a Subject is what that action or expression concerns. 02 §3.2 · Subject Is Not Actor
Actor ≠ Participant
Acting is about responsibility for an action, while participating is about taking part in a bounded context; one Person may be both at once. 01 §3.1 · Subject Is a Role